Isomerization of a light hydrocarbon oil is a technique which has conventionally been employed extensively in the field of petroleum refining industry and petrochemical industry. With the trend toward engine performance elevation in motor vehicles and aircraft especially in recent years, the gasoline for use as a fuel therein have come to be required to have a high octane number and hence the isomerization is becoming important for satisfying the requirement. The so-called isomerized gasoline has hitherto been used as one of light blend stocks for gasoline, the isomerized gasoline being obtained by isomerizing a light naphtha that is a light hydrocarbon oil so as to have an improved octane number.
Many researches have conventionally been conducted on methods for isomerizing light naphthas, and various catalysts for use in the isomerization reactions are known. Of these, solid acid catalysts can be mentioned as the most useful isomerization catalysts. Processes for producing solid acid catalysts and methods of isomerization using the catalysts are disclosed, for example, in JP-B-5-29503 and JP-B-6-29199.
However, light naphthas obtained by distillation or cracking of crude oil, usually contain organosulfur compounds in an amount of about 200 to 700 ppm and these organosulfur compounds function as a catalyst poison for solid acid catalysts. Therefore, direct isomerization of light naphthas has not been a process suitable for industrial use in view of catalyst life. In processes which are presently carried out, a two-stage treatment is conducted, wherein a light naphtha containing sulfur compounds is treated with a Co—Mo based or Ni—Mo based hydrodesulfurization catalyst to convert organosulfur compounds into hydrogen sulfide and the hydrogen sulfide is separated from the product oil to thereby prepare a desulfurized light naphtha having a sulfur content reduced to several ppm or lower; and thereafter this desulfurized light naphtha is used as a feedstock oil to be isomerized. Namely, in the current processes for the isomerization of a light hydrocarbon oil, the step of hydrodesulfurization is indispensable as a pretreatment for the step of isomerization.
If a catalyst for use in the isomerization of a light hydrocarbon oil can be replaced with a catalyst capable of simultaneously achieving hydrodesulfurization and isomerization, the hydrodesulfurization step which has been indispensable to isomerization processes can be omitted, making it possible to conduct the isomerization more economically with simpler facilities than in conventional techniques. Specifically, it is desirable to enable hydrodesulfurization and isomerization reactions to be conducted simultaneously by loading an isomerization catalyst having sulfur tolerance into an existing reaction column for the isomerization of a light hydrocarbon oil and feeding a light hydrocarbon oil containing organosulfur compounds as a feedstock oil to be isomerized.
As a catalyst satisfying such a requirement, recently disclosed is a catalyst in which sulfuric acid and a transition metal such as platinum are incorporated into a molded form of zirconia and alumina (WO00/12652). However, the sulfur tolerance thereof is not sufficiently high.
The present inventors have also studied catalysts of sulfated zirconia in combination with a platinum group element. They have found catalysts capable of simultaneously achieving the desulfurization and isomerization of a light hydrocarbon and thus proposed methods for hydrodesulfurization and isomerization using the catalysts (JP-A-2000-233132 and JP-A-2000-234093), but a catalyst having higher activity and higher sulfur tolerance has been still required.